The present invention relates to crankcase scavenged two-stroke engines and is particularly, though not exclusively, concerned with small engines of this type which are intended for use on hand-held products such as chain saws, garden blowers and the like.
The cylinder of a crankcase scavenged two-stroke engine includes an inlet port, an outlet port and a transfer port which are arranged so that the exhaust port opens before and closes after the transfer port. The transfer port is essentially one or more transfer passages which connect the cylinder and crankcase and are arranged in such a way that the piston and the cylinder controls the opening and closing of the downstream end of the transfer passages during the engine cycle. This type of engine has a hermetically sealed crankcase which communicates with the cylinder via the transfer port and with the atmosphere via an inlet duct. As the piston performs its cylinder compression stroke, air or an air/fuel mixture is drawn into the crankcase from the atmosphere through the inlet duct and on the subsequent working stroke this air or air/fuel mixture is compressed by the piston. As the piston continues to move it uncovers the downstream end of the transfer port and the air or air/fuel mixture is forced into the cylinder.
The transfer of air or air/fuel mixture into the cylinder only occurs when a positive pressure differential exists between the crankcase and cylinder. The fresh charge of air or air/fuel mixture entering the cylinder causes the displacement of residual gas from the cylinder via the exhaust port. During this cylinder scavenging process a portion of the air or air/fuel mixture that has entered the cylinder flows out of the cylinder via the exhaust port. The charge lost in this way is usually termed the scavenge losses. This loss of charge can also occur during the period in the engine cycle between transfer port closure and exhaust port closure. This period is known as the trapping period and the associated losses are usually termed the trapping losses.
Two-stroke engines of the type which are fitted on to small motorcycles, scooters and the like are typically provided with a carburettor which is arranged to dispense fuel into the inlet duct in an amount which is related to the air flow rate through that duct. This means that all the air/fuel mixture which enters the crankcase and subsequently the cylinder is inherently a substantially homogeneous mixture of air and fuel. This means in turn that the proportion of the scavenging air which flows out of the exhaust port also contains fuel. This results in the unburned hydrocarbon emissions of such engines being relatively high.
Small two-stroke engines, particularly those for use with hand-held products, are facing ever stricter emission control legislation and durability requirements. Yet stricter legislation is expected in the USA in the near future and this legislation will be particularly severe for such small engines and will include limits not only on unburned hydrocarbons (HC) and carbon monoxide (CO) but also particulate emissions. No currently available small two-stroke engine is able to meet the requirements which will be introduced in the USA without emissions control equipment, such as an oxidation catalyst. It should also be noted that with small engines of this type there can be a variation of up to 25% in HC emissions from two engines which are nominally identical. Engine manufacturers are, however, reluctant to use catalysts and/or other potentially costly emissions control equipment and require a solution to the problem that has minimal or zero cost implications. If a catalyst is still required after the implementation of other emissions reducing technology, the load on the catalyst must be minimised in order to reduce the size and cost of the catalyst, to minimise any increase in the exhaust gas temperature and to improve the durability of the catalyst.
The emissions performance of two-stroke engines under high load, i.e. when the throttle is wide open, is crucial for the ability of such engines to obtain certification under emissions control legislation, particularly for those engines which are intended for use with hand-held equipment. It is also under high load that the maximum catalyst/exhaust gas temperatures are reached and at which maximum thermal degradation of the catalyst occurs. Accordingly, any attempt to reduce the emissions of an engine should focus on the emissions at high load, emissions at low load being of substantially lesser importance.
Given the major significance of the emissions at high load for two-stroke engines for use with hand-held equipment, there are in practice only two types of technology that could realistically reduce the HC emissions at high load to acceptable levels, namely catalytic after-treatment and stratified charging. Catalytic after-treatment, comprising subjecting the exhaust gases to an oxidation catalyst, has been referred to above. A catalyst may also be required to reduce CO emission but it is believed that if the engine is adjusted to run with a leaner mixture it may be possible to meet the requirements of the anticipated US legislation without a catalyst. It may be possible also to meet the anticipated legislative requirements relating to HC emissions with a catalyst but the service life of the catalyst may well cause a problem unless the loading to which the catalyst is subjected can be reduced by reducing the HC content of the exhaust gas leaving the engine cylinders. It will be appreciated that reducing the HC loading on the catalyst will reduce the size and cost of the catalyst, minimise the heat added to the exhaust gas by the catalysis, increase the service life of the catalyst and reduce the influence of the catalyst on the exhaust tuning. Stratified charging constitutes, as is known, arranging the inlet system of the engine such that the air/fuel charge entering the cylinder is nonhomogeneous in such a manner that substantially only pure air and a minimum quantity of fuel is permitted to pass directly from the cylinder into the exhaust port during the scavenging and trapping processes.
This may be achieved by providing the engine with direct fuel injection, that is to say a fuel injector which communicates directly with the cylinder and is controlled by an electronic control system which is arranged to ensure that the correct amount of fuel is injected into the cylinder after the exhaust port has closed. Whilst effective, this solution to the problem is expensive due to the need to provide a speed and load-responsive electronic control system and a fuel injector and is thus unacceptable in small, low cost engines.
GB-A-2290349 discloses a further attempt to solve this problem. This specification discloses a crankcase scavenged engine with so-called transfer port stratified charging. The engine disclosed in this prior document includes a transfer port constituted by two or more transfer passages, into only one of which is fuel dispensed. The other or others of the transfer passages communicate with the interior of the cylinder at a position which is further from the crankshaft axis. In use, fuel is dispensed into the one transfer passage substantially continuously at a rate which is a function of the mass flow rate of air through the inlet passage into the crankcase. As the piston performs its exhaust stroke, the exhaust port is the first to be uncovered by the piston and thereafter the other transfer passage or passages are uncovered. Shortly thereafter, the transfer passage into which fuel is dispensed is uncovered and the air/fuel charge flows into the cylinder. However, scavenging is performed predominantly with the pure air which has flowed in through the other transfer passage or passages whereby the amount of unburned fuel which passes straight through the cylinder into the exhaust port during the scavenging process is reduced.
Tests have shown that an engine of the type disclosed in GB-A-2290349 has unburned HC emissions under high load conditions which are reduced by about 50%, as compared to conventional engines. However, as the engine load decreases, the reduction in unburned HC emissions decreases also until at about 40% throttle opening there is no net improvement. As the throttle is closed yet further, the unburned HC emissions are actually increased by comparison with a homogeneously charged two-stroke engine. The reason for this is believed to be that a low engine loads a proportion of the air/fuel charge flows in a direct short circuit across the top of the piston directly to the exhaust port.
A further significant problem with the engine disclosed in GB-A-2290349 relates to the fuel dispensing device which is considerably different to a conventional carburettor. Thus the deviation from known carburettor technology means that the fuel dispensing device is significantly more expensive to manufacture and it has in any event been found in practice that it is very difficult to design a fuel dispensing device which delivers the correct quantity of fuel over the entire engine operating range.
It is therefore that the solution to the emissions problems described above will rely upon using stratified charging at high engine load but homogeneous charging at lower engine load. It is also thought that it is necessary for commercial success for the engine to use a more conventional carburettor.